1) Field of the Invention
The present invention relates to a photo-sensitive resin composition, a process for producing a polyamide resin for use in the composition, a photosensitive element, a photosensitive laminate, and a process for producing a flexible printed circuit board.
2) Description of Related Art
For protection of an outerlayer circuit of a flexible printed circuit board, a coverlay formed by die-cutting a polyimide film, followed by heated pressure bonding or a cover coat formed by screen printing a polyimide resin ink has been so far used. The coverlay is limited in die-cut pattern micronization and thus is not applicable to a solder dam between QFP (quad flat package) pads at a narrower pitch. Furthermore, the coverlay is also limited in positional precision of lamination and thus there is a necessity for circuit designs with an allowance for a possible positional deviation. That is, the coverlay has such a problem as a failure to make FPC (flexible printed circuit board) higher in density and small in size. The cover coat, on the other hand, is limited likewise in pattern micronization due to the screen printing and also has such a problem as poor workability.
To overcome these problems, JP-A-64-2037, JP-A-64-484893, JP-A-5-158237 and JP-A-6-298935 propose processes for forming a coverlay with finer patterns by forming a photosensitive resin composition comprising a poly(amic acid)(polyimide precursor), a photocross-linking agent having an ethylenically unsaturated group and a photopolymerization initiator on a flexible printed circuit board, followed by light exposure, development and heating. However, the proposed processes have such problems as a failure to give the coverlay a necessary and sufficient thickness in respect to light transmission, a necessity for an additional step of heating to a temperature of 250.degree. C. or higher to conduct ring closure of poly(amic acids) for forming an ultimate polyimide skeleton, inevitable oxidation of copper circuits, deterioration of adhesive layers, warping of substrate, etc.
Furthermore, JP-A-5-183260 proposes a process, which comprises forming a coverlay in double layer structure of thermoplastic polyimide and non-thermoplastic polyimide on a flexible printed circuit board by heated pressure bonding, forming a resist pattern thereon by screen printing and patterning the coverlay by alkali etching. JP-A-5-254064 proposes a process, which comprises laminating a double layer type film of a poly(amic acids)(polyimide precursor) layer and a photosensitive resin layer on a flexible printed circuit board, at first subjecting the photosensitive resin layer to light exposure and development to form a pattern, and successively patterning the poly(amic acid) layer by alkali etching, using the photosensitive resin layer as an etching resist. However, the former process is limited in pattern micronization due to the screen printing and also has such problems as a poor workability and a high cost due to the double layer structure of polyimide film. The latter process has such a work-ability problem as a necessity for the so called wet lamination and a separate development process each for the photosensitive resin layer and the poly(amic acid) layer, and furthermore has such problems as a necessity for an additional step for heating to a temperature of 250.degree. C. or higher to conduct ring closure of poly(amic acid) for forming a final polyimide skeleton, inevitable oxidation of copper circuits, deterioration of adhesive layers, warping of substrate, etc.
From viewpoints of cost and environmental sanitation, it is preferable to use an aqueous inorganic alkali solution free from organic solvents as a developing solution, but almost all developing solutions applicable to the foregoing resin materials are organic solvents or semi-organic solvent systems based on mixtures of an organic solvent and an aqueous alkali solution.
On the other hand, JP-A-56-6498 and JP-A-59-230014 propose a process for producing a flexible printed circuit board, using an acrylic resin-based, photosensitive film so far employed in the field of printed wiring boards as a coverlay, where the coverlay can be readily formed at a low cost by a similar process to that for a dry film type permanent mask resist so far employed in the field of printed wiring (or circuit) boards, but both satisfactory flexibility and heat resistance or both satisfactory flexibility and solvent resistance cannot be obtained at the same time. JP-A-6-332171 proposes use of a photosensitive film based on a combination of acrylic resin and a specific urethane monomer as a coverlay film, where no satisfactory solvent resistance and electrical reliability can be obtained. No satisfactory nonflammability can be obtained with acrylic resin based, photosensitive films.